1. Field of the Invention
This invention generally relates to gas variable-power boilers. More particularly, this invention relates to a gas injection pipe at the burner arms which enables the gas to be variably supplied thereto.
2. Description of the Relevant Art
In gas heating devices and more precisely in the so-called "mixed" devices (i.e., the ones that operate both in central heating and in the supply of hot sanitary water), it is often required to modulate the power of any device with respect to the heating requirements for the relative flat, and it is for instance known that the required power is limited as to heating purposes, whereas it can be maximum with respect to the feeding thereof.
Mixed gas boilers intended to be affixed on walls are known, wherein a progressive regulation enables the gas flow rate at the burner to be so monitored that the power supplied by the boiler can be matched as close as possible to the heating needs.
Unfortunately, in the known structures of these boilers, the regulation range has to be downwardly limited at about a third of the nominal power of the relative boiler, since the yield drop of the boilers with respect to the relative decrease of the gas flow rate becomes substantially inadequate below the third portion of the nominal flow rate.
Since it is impossible to regulate the boiler power below 30%, this results in a drawback which is increasingly detrimental for mixed boilers at a time where the powers set up for heating houses would tend to decrease and where the powers for producing hot sanitary water would on the contrary tend to increase; thus, involving an increasingly large disparity between the power to be supplied when heating and the outlet power.
In order to avoid the above-discussed drawbacks and set up the regulation zone of the device as large as possible, the pipe on which are mounted the burner injectors is shared so as to provide at least two distinct injection pipes, one being fed so as to heat, whereas the other one are outlet-supplied, but this structural arrangement requires an additional electrovalve to be mounted on the usual gas equipment.
In variable-power devices, it is now preferable to use electrovalves having a progressive openings; thus, enabling when starting from a first opening threshold corresponding to a determined flow rate, a progressive increasing of the gas inlet rate; i.e., directly monitored by the intensity of the energizing current of the electrovalve solenoid. However, except the known drawbacks inherent to the mounting and adjustment of the electrovalves, it has appeared that, due to hysteresis problems, the opening of the profiled valve opening is not easily controlled.
In fact, whether or not the electrovalve solenoid is energized by increasing or decreasing intensities of the electric current, due to the hysteresis, the opening of the profiled valve for an even intensity is not identical, but depends whether or not the current is either in its rising phase or in its descending phase. As a result, the gas flow rate supplied to the burner is modified. Since the oxidizing air is shared into a primary air supplied to the burner arm and a secondary air, the gas regulation by means of a conventional electrovalve only substantially modifies the primary air flow rate, whereas the secondary air flow rate is kept substantially uniform, and as a result, the combustion yield is substantially lowered.
Independent from the hysteresis, the accuracy of the gas flow rate to be fed is directly bound to the accuracy of the progressive opening of the profiled valve and thus, it is very sensitive to size variations of the valve and the adjustment thereof as well.